Effects of frost during grain filling on wheat yield and grain structure

A late frost occurred in the Southland region of New Zealand in November 1995 when many winter wheat (Triticum aestivum L.) crops were at early grain fill, and resulted in a large number of insurance claims for frost damage. In this study, frost damage was characterised. Spikes in affected crops were sharply tapered from a point half to two-thirds from their bases. Grains were as much as 80% lighter in upper parts of affected spikes. Overall yield losses as a result of frost of between 13 and 33% were calculated in affected crops. Grains were sectioned and examined with a scanning electron microscope. Whereas in healthy grains, the layers making up the pericarp and testa were compressed, in frosted grains these layers comprised loosely compressed and unstructured networks of cells. The aleurone layer was less ordered in frosted than in normal grains, and was not always readily distinguished from the starchy endosperm. Rehydration of frosted grains was accompanied by expansion of pericarp and testa cells, whereas little cell expansion occurred in normal grains. Blisters were common in frosted grains, where the outer layers were detached from the remainder of the pericarp.